This invention relates to coatings for the bearing surfaces of piston rings, particularly piston rings used in an internal combustion engine. The coatings are produced by plasma spray application of a powder to the bearing face of the piston ring, which powder is composed of oxides of titanium, aluminum and yttrium.
Plasma applied coatings of alumina and titania for the bearing faces of compression and oil control piston rings of internal combustion engines are known. Thus, U.S. Pat. No. 3,697,091, describes a bearing face coating which consists essentially of about 75-90% aluminum oxide and 10-25% by weight titanium oxide. A piston ring coating of alumina and titania along with ferric oxide is disclosed in U.S. Pat. No. 4,077,637. In U.S. Pat. No. 4,115,959, an alumina-titania coating is described which further includes about 10-15% of an alkaline earth metal fluoride which reduces the wear on grinding wheels used to grind and finish the coated piston rings. The inclusion of the alkaline earth metal fluoride has been found to reduce the number of dressing operations required to be performed on the grinding wheels during the finish grinding process.
These refractory metal oxide coatings are superior to various metal and metal alloy coatings, such as molybdenum alloy coatings, in a number of respects. Specifically, the metal oxide coating compositions have been found to have improved scuff resistance and outstanding thermal shock resistance when compared to molybdenum alloy coatings.
Rings coated with alumina-titania plasma applied coatings have exhibited a tendency to flake or blister during engine operation. It is believed that at elevated temperatures such as found in a diesel engine, the metal oxide coating materials undergo a phase transformation. This phase transformation is believed to cause loss of intra-coating cohesion resulting in undesirable delamination by blistering and spalling of portions of the coating. The delamination is due to lack of cohesion within the coating itself. Blisters of about 1/16" diameter and 0.0001" thickness appear in the surface of the coating which is generally 0.004" thick. The blister material is then scuffed off and a loss of coating results. Another theory is that Hertzian stress caused by contact in the cylinder will cause cracks to develop within the coating layers resulting in flaking delamination.
It is known in the art that yttrium (as the element) aids in the bonding of naturally formed oxide coatings to the surface of metal superalloys. In a paper presented in the International Conference on Metallurgical Coatings in San Francisco in April, 1976, entitled "Plasma Spraying of Al.sub.2 O.sub.3 and Al.sub.2 O.sub.3 -Y.sub.2 O.sub.3 " ("Thin Solid Films"39 (1976) pp. (251-262), it was reported that yttrium oxide has similar beneficial effects on bonding adhesion of sprayed oxide coatings, specifically aluminum oxide coatings, over a steel substrate.